Many battery charge indicators employ a thermochromatic material as the display or meter for indicating the relative charge on a battery. These indicators have been integrated into the packaging of batteries and have even been mounted directly on the batteries themselves. In this type of battery charge indicator, thermochromatic ink is applied over a tapered resistive strip that produces heat when the battery voltage is applied to the resistive strip, causing the current to flow through the strip. The heat causes the thermochromatic ink to react by becoming clear and revealing an underlying graph to give a visual indication of the charge on the battery.
These indicators generally work well to display the charge on a batter which has a relatively wide operating voltage range, for example, approximately 0.5 V or more, and they can be connected directly to the battery terminals. Alkaline batteries or series packs of three or more nickel-cadmium (Ni--Cd) batteries have a sufficiently wide operating voltage range such that the changes in the voltage level as the battery is discharged or charged is adequately displayed on the thermochromatic material. In other words, the variation of voltage from one end of the range to the other is sufficiently wide so that the thermochromatic ink in the thermochromatic indicator reveals the underlying graph over a substantial portion of the thermochromatic material.
However, for batteries such as a single rechargeable Ni--Cd battery or a nickel metal hydride battery which have a relatively small or narrow operating voltage range between fully charged and discharged conditions, normally about 0.2 V, the conventional thermochromatic indicators connected directly to the battery terminals have been unable to indicate the battery charge level with adequate resolution. The portion, of the thermochromatic ink affected by the change in the battery voltage from one end of the operating range to another is only a fraction of the entire thermochromatic material. Therefore, an incremental or a decremental change in the battery voltage is too small to significantly vary the current through the resistive strip. This results in a correspondingly small change in the heat produced by the resistive strip and similarly negligible portion of the thermochromatic material being affected by the change. Consequently, the variation in the battery charge level is almost indistinguishable to a user viewing the thermochromatic material for the purposes of determining the charge on the battery because of poor resolution.
Thus, there is a need for apparatus for indicating a charge with good resolution when the battery has a relatively small operating voltage range between the charged and discharged conditions.
Accordingly, one object of this invention is to provide a new and improved battery charge indicator.
Another object is to provide a new and improved battery charge indicator having high resolution when the battery has a relatively small voltage range between the charged and discharged conditions.